Typically, polyurethane adhesives are made by combining a prepolymer component with a curative component just before use. The adhesive is then applied to a car part made of fiberglass reinforced polyester (FRP) and a second part is placed over the adhesive. The adhesive is then cured by placing the adhered parts in an oven maintained at 300.degree. F. (150.degree. C.) for 30 minutes to cure or harden the adhesive.
In order to be commercially useful a structural polyurethane adhesive must demonstrate a number of physical properties. The best adhesives of this type are: primerless, non-sag, have a long open time, cure at lower temperatures than 300.degree. F. (150.degree. C.), and are so strong that, when bonds are broken, the substrate delaminates or fails before the adhesive itself fails.
Although primers are usually used to treat the substrates before applying polyurethane adhesives, it is better to eliminate the primer step if possible because primers often contain chlorinated hydrocarbons and because eliminating any single step in a multiple step process is an improvement.
Often the adhesive must be applied to a substrate in a vertical position where, if the adhesive is too fluid, it will drip or sag before the second substrate can be placed over the adhesive. Thus, an adhesive which does not sag is preferred. This property is known as "non-sag" and is measured by noting the distance the adhesive flows at the end of a period of time such as three minutes. The test method used to measure sag is ASTM D2202-84. Instead of a cylindrical cavity used for the test in ASTM D2202-84, sometimes a bead cavity of 0.95 cm wide.times.7.64 cm long and 0.63 cm deep is used to sample the adhesive. In this case, the results are called bead sag, to differentiate from the button sag results of the unmodified test.
It is important that an adhesive remain uncured and fluid for sufficient time to permit placing of the second substrate onto the adhesive. An adhesive which hardens too quickly does not permit flexibility in the assembly line process. Thus, the length of time the adhesive is fluid is measured and is referred to as "open time".
If an adhesive requiring 300.degree. F. (150.degree. C.) cure could be replaced by a strong adhesive curing at 240.degree. F. (116.degree. C.), this lower temperature cure will save energy and permit lower temperature bake cycles of the subsequent painting step.
The most preferred adhesives for FRP will be curable at 240.degree. F. (116.degree. C.) and will show a failure of the substrate at 180.degree. F. when force is applied to separate the adhered parts. When the adhesive itself fractures the result is called "adhesive failure" (AF). When the substrate itself breaks the result is called "delamination" (DL). When an adhesive is said to have exhibited 100% substrate delamination the entire broken interface between adhesive and substrate shows only the torn fibers and disrupted structure of the substrate itself.
Previously these polyurethane structural adhesives have been cured by baking at least at 300.degree. F. (150.degree. C.) for 30 minutes. These adhesives usually required the use of a primer and consisted of two parts: an isocyanate terminated prepolymer and a curative made from a polyether polyol, a diamine and an optional catalyst. When the automobile industry demanded adhesives curing at lower temperatures so as to save energy it was necessary to develop new adhesive chemistry because these conventional adhesives, curable only at 300.degree. F. (150.degree. C.) could not show adequate substrate delamination at 180.degree. F. (82.degree. C.) after curing at 240.degree. F. (116.degree. C.) for an hour. New adhesive chemistry also had to be useful on substrates in any environment all year including humid conditions. Such adhesives are said to be "water resistant".